scholarly journals Northern Hemispheric Interdecadal Variability: A Coupled Air–Sea Mode

1998 ◽  
Vol 11 (8) ◽  
pp. 1906-1931 ◽  
Author(s):  
A. Timmermann ◽  
M. Latif ◽  
R. Voss ◽  
A. Grötzner
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Temperature Pattern ◽  
The North ◽  
Northern Hemispheric ◽  
The North Atlantic ◽  
Climate Mode ◽  
Sst Anomalies

Abstract A coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific. The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.

scholarly journals Effect of vegetation on the Late Miocene ocean circulation

2006 ◽  
Vol 2 (4) ◽  
pp. 605-631 ◽  
Author(s):  
G. Lohmann ◽  
M. Butzin ◽  
A. Micheels ◽  
T. Bickert ◽  
V. Mosbrugger
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Late Miocene ◽  
Land Surface ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Hydrological Cycle ◽  
Circulation Model ◽  
The North ◽  
The North Atlantic

Abstract. A weak and shallow thermohaline circulation in the North Atlantic Ocean is related to an open Central American gateway and exchange with fresh Pacific waters. We estimate the effect of vegetation on the ocean general circulation using the atmospheric circulation model simulations for the Late Miocene climate. Caused by an increase in net evaporation in the Miocene North Atlantic, the North Atlantic water becomes more saline which enhances the overturning circulation and thus the northward heat transport. This effect reveals a potentially important feedback between the ocean circulation, the hydrological cycle and the land surface cover for Cenozoic climate evolution.

scholarly journals Nonnormal Thermohaline Circulation Dynamics in a Coupled Ocean–Atmosphere GCM

2008 ◽  
Vol 38 (3) ◽  
pp. 588-604 ◽  
Author(s):  
Eli Tziperman ◽  
Laure Zanna ◽  
Cecile Penland
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Initial Perturbation ◽  
Circulation Model ◽  
Empirical Orthogonal Functions ◽  
Transient Growth ◽  
The North ◽  
The North Atlantic ◽  
Reduced Space

Abstract Using the GFDL coupled atmosphere–ocean general circulation model CM2.1, the transient amplification of thermohaline circulation (THC) anomalies due to its nonnormal dynamics is studied. A reduced space based on empirical orthogonal functions (EOFs) of temperature and salinity anomaly fields in the North Atlantic is constructed. Under the assumption that the dynamics of this reduced space is linear, the propagator of the system is then evaluated and the transient growth of THC anomalies analyzed. Although the linear dynamics are stable, such that any initial perturbation eventually decays, nonnormal effects are found to result in a significant transient growth of temperature, salinity, and THC anomalies. The growth time scale for these anomalies is between 5 and 10 yr, providing an estimate of the predictability time of the North Atlantic THC in this model. There are indications that these results are merely a lower bound on the nonnormality of THC dynamics in the present coupled GCM. This seems to suggest that such nonnormal effects should be seriously considered if the predictability of the THC is to be quantitatively evaluated from models or observations. The methodology presented here may be used to produce initial perturbations to the ocean state that may result in a stricter estimate of ocean and THC predictability than the common procedure of initializing with an identical ocean state and a perturbed atmosphere.

scholarly journals North Atlantic SSTs as a Link between the Wintertime NAO and the Following Spring Climate

2014 ◽  
Vol 27 (1) ◽  
pp. 186-201 ◽  
Author(s):  
Ivana Herceg-Bulić ◽  
Fred Kucharski
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Theoretical Physics ◽  
Storm Activity ◽  
Climate Anomalies ◽  
The North ◽  
Temperature And Precipitation ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract In this paper a potential seasonally lagged impact of the wintertime North Atlantic Oscillation (NAO) on the subsequent spring climate over the European region is explored. Supported by the observational indication of the wintertime NAO–spring climate connection, a modeling approach is used that employs the International Centre for Theoretical Physics (ICTP) atmospheric general circulation model (AGCM) as a stand-alone model and that is also coupled with a mixed layer ocean in the North Atlantic. Both observational and modeled data indicate a pattern of sea surface temperatures (SSTs) in North Atlantic as a possible link between wintertime NAO and climate anomalies in the following spring. The SST pattern is associated with wintertime NAO and persists through the following spring. It is argued that these SST anomalies can affect the springtime atmospheric circulation and surface conditions over Europe. The atmospheric response is recognized in observed as well as in modeled data (mean sea level pressure, temperature, and precipitation). Additionally, an impact on springtime storm activity is found as well. It is demonstrated that the SST anomalies associated with wintertime NAO persist into the subsequent spring. These SST anomalies enable atmosphere–ocean interaction over the North Atlantic and consequently affect the climate variability over Europe. Although it has a relatively weak impact, the described mechanism provides a temporal teleconnection between the wintertime NAO and subsequent spring climate anomalies.

scholarly journals Marine productivity response to Heinrich events: a model-data comparison

2012 ◽  
Vol 8 (5) ◽  
pp. 1581-1598 ◽  
Author(s):  
V. Mariotti ◽  
L. Bopp ◽  
A. Tagliabue ◽  
M. Kageyama ◽  
D. Swingedouw
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Global Ocean ◽  
Biogeochemical Model ◽  
Heinrich Events ◽  
The North ◽  
Heinrich Event ◽  
The North Atlantic ◽  
Marine Productivity

Abstract. Marine sediments records suggest large changes in marine productivity during glacial periods, with abrupt variations especially during the Heinrich events. Here, we study the response of marine biogeochemistry to such an event by using a biogeochemical model of the global ocean (PISCES) coupled to an ocean-atmosphere general circulation model (IPSL-CM4). We conduct a 400-yr-long transient simulation under glacial climate conditions with a freshwater forcing of 0.1 Sv applied to the North Atlantic to mimic a Heinrich event, alongside a glacial control simulation. To evaluate our numerical results, we have compiled the available marine productivity records covering Heinrich events. We find that simulated primary productivity and organic carbon export decrease globally (by 16% for both) during a Heinrich event, albeit with large regional variations. In our experiments, the North Atlantic displays a significant decrease, whereas the Southern Ocean shows an increase, in agreement with paleo-productivity reconstructions. In the Equatorial Pacific, the model simulates an increase in organic matter export production but decreased biogenic silica export. This antagonistic behaviour results from changes in relative uptake of carbon and silicic acid by diatoms. Reasonable agreement between model and data for the large-scale response to Heinrich events gives confidence in models used to predict future centennial changes in marine production. In addition, our model allows us to investigate the mechanisms behind the observed changes in the response to Heinrich events.

The Effect of a Large Freshwater Perturbation on the Glacial North Atlantic Ocean Using a Coupled General Circulation Model

2006 ◽  
Vol 19 (17) ◽  
pp. 4436-4447 ◽  
Author(s):  
C. D. Hewitt ◽  
A. J. Broccoli ◽  
M. Crucifix ◽  
J. M. Gregory ◽  
J. F. B. Mitchell ◽  
...  
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Quasi Equilibrium ◽  
Last Glacial ◽  
Proxy Records ◽  
The North ◽  
The North Atlantic ◽  
The Last Glacial

Abstract The commonly held view of the conditions in the North Atlantic at the last glacial maximum, based on the interpretation of proxy records, is of large-scale cooling compared to today, limited deep convection, and extensive sea ice, all associated with a southward displaced and weakened overturning thermohaline circulation (THC) in the North Atlantic. Not all studies support that view; in particular, the “strength of the overturning circulation” is contentious and is a quantity that is difficult to determine even for the present day. Quasi-equilibrium simulations with coupled climate models forced by glacial boundary conditions have produced differing results, as have inferences made from proxy records. Most studies suggest the weaker circulation, some suggest little or no change, and a few suggest a stronger circulation. Here results are presented from a three-dimensional climate model, the Hadley Centre Coupled Model version 3 (HadCM3), of the coupled atmosphere–ocean–sea ice system suggesting, in a qualitative sense, that these diverging views could all have occurred at different times during the last glacial period, with different modes existing at different times. One mode might have been characterized by an active THC associated with moderate temperatures in the North Atlantic and a modest expanse of sea ice. The other mode, perhaps forced by large inputs of meltwater from the continental ice sheets into the northern North Atlantic, might have been characterized by a sluggish THC associated with very cold conditions around the North Atlantic and a large areal cover of sea ice. The authors’ model simulation of such a mode, forced by a large input of freshwater, bears several of the characteristics of the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) Project’s reconstruction of glacial sea surface temperature and sea ice extent.

scholarly journals Modelling the wintertime response to upper tropospheric and lower stratospheric ozone anomalies over the North Atlantic and Europe

2003 ◽  
Vol 21 (10) ◽  
pp. 2107-2118 ◽  
Author(s):  
I. Kirchner ◽  
D. Peters
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Initial Conditions ◽  
Stratospheric Ozone ◽  
Circulation Model ◽  
Sensitivity Study ◽  
Boreal Winter ◽  
Radiative Processes ◽  
The North ◽  
The North Atlantic

Abstract. During boreal winter months, mean longitude-dependent ozone changes in the upper troposphere and lower stratosphere are mainly caused by different ozone transport by planetary waves. The response to radiative perturbation induced by these ozone changes near the tropopause on the circulation is unclear. This response is investigated with the ECHAM4 general circulation model in a sensitivity study. In the simulation two different mean January realizations of the ozone field are implemented in ECHAM4. Both ozone fields are estimated on the basis of the observed mean January planetary wave structure of the 1980s. The first field represents a 14-year average (reference, 1979–1992) and the second one represents the mean ozone field change (anomaly, 1988–92) in boreal extra-tropics during the end of the 1980s. The model runs were carried out pairwise, with identical initial conditions for both ozone fields. Five statistically independent experiments were performed, forced with the observed sea surface temperatures for the period 1988 to 1992. The results support the hypothesis that the zonally asymmetric ozone changes of the 80s triggered a systematic alteration of the circulation over the North Atlantic – European region. It is suggested that this feedback process is important for the understanding of the decadal coupling between troposphere and stratosphere, as well as between subtropics and extra-tropics in winter.Key words. Meteorology and atmospheric dynamics (general circulation; radiative processes; synoptic-scale meteorology)

scholarly journals Large-Scale Controls on Atlantic Tropical Cyclone Activity on Seasonal Time Scales

2016 ◽  
Vol 29 (18) ◽  
pp. 6727-6749 ◽  
Author(s):  
Young-Kwon Lim ◽  
Siegfried D. Schubert ◽  
Oreste Reale ◽  
Andrea M. Molod ◽  
Max J. Suarez ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
North Atlantic ◽  
General Circulation ◽  
Large Scale ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Circulation Model ◽  
The North ◽  
Predictive Skill ◽  
The North Atlantic

Abstract Interannual variations in seasonal tropical cyclone (TC) activity (e.g., genesis frequency and location, track pattern, and landfall) over the Atlantic are explored by employing observationally constrained simulations with the NASA Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model. The climate modes investigated are El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Atlantic meridional mode (AMM). The results show that the NAO and AMM can strongly modify and even oppose the well-known ENSO impacts, like in 2005, when a strong positive AMM (associated with warm SSTs and a negative SLP anomaly over the western tropical Atlantic) led to a very active TC season with enhanced TC genesis over the Caribbean Sea and a number of landfalls over North America, under a neutral ENSO condition. On the other end, the weak TC activity during 2013 (characterized by weak negative Niño index) appears caused by a NAO-induced positive SLP anomaly with enhanced vertical wind shear over the tropical North Atlantic. During 2010, the combined impact of the three modes produced positive SST anomalies across the entire low-latitudinal Atlantic and a weaker subtropical high, leading to more early recurvers and thus fewer landfalls despite enhanced TC genesis. The study provides evidence that TC number and track are very sensitive to the relative phases and intensities of these three modes and not just to ENSO alone. Examination of seasonal predictability reveals that the predictive skill of the three modes is limited over tropics to subtropics, with the AMM having the highest predictability over the North Atlantic, followed by ENSO and NAO.

scholarly journals North Atlantic SST Anomalies and the Cold North European Weather Events of Winter 2009/10 and December 2010

2014 ◽  
Vol 142 (2) ◽  
pp. 922-932 ◽  
Author(s):  
Jian Buchan ◽  
Joël J.-M. Hirschi ◽  
Adam T. Blaker ◽  
Bablu Sinha
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Cold Weather ◽  
Global Ocean ◽  
Northern Europe ◽  
Significant Shift ◽  
The North ◽  
Extreme Cold ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract Northern Europe experienced consecutive periods of extreme cold weather in the winter of 2009/10 and in late 2010. These periods were characterized by a tripole pattern in North Atlantic sea surface temperature (SST) anomalies and exceptionally negative phases of the North Atlantic Oscillation (NAO). A global ocean–atmosphere general circulation model (OAGCM) is used to investigate the ocean’s role in influencing North Atlantic and European climate. Observed SST anomalies are used to force the atmospheric model and the resultant changes in atmospheric conditions over northern Europe are examined. Different atmospheric responses occur in the winter of 2009/10 and the early winter of 2010. These experiments suggest that North Atlantic SST anomalies did not significantly affect the development of the negative NAO phase in the cold winter of 2009/10. However, in November and December 2010 the large-scale North Atlantic SST anomaly pattern leads to a significant shift in the atmospheric circulation over the North Atlantic toward a NAO negative phase. Therefore, these results indicate that SST anomalies in November/December 2010 were particularly conducive to the development of a negative NAO phase, which culminated in the extreme cold weather conditions experienced over northern Europe in December 2010.

Sign in / Sign up

Export Citation Format

Share Document